Modern man's need for water is pressing. As the world's population has increased, so has the demand for water, especially for pure water, to safeguard public health. From 1900 to 1960 the demand for water in the United States increased six-fold. By the year 2001, the demand for water averaged 900,000,000,000 (billion) gallons per day. Next to homes and agriculture, industries are major users of water.
There are six different kinds of freshwater reserves, or sources. The four concentrated (or main) reserves are those in rivers, lakes, man-made reservoirs, and groundwater. The other two water reservoirs are found in the atmosphere and the soil. These sources are so difficult to tap that they are known as diffuse, or scattered reserves.
Water engineers are dealing, and have experimented with various methods, including converting salt water into fresh water. The principle methods researched, developed and so far put to limited use are distillation (or evaporation), electrodialysis, freezing, ion exchange, and reverse osmosis. A number of large desalination plants, some with capacities of 1,000,000 or more gallons per day, have been constructed in the United States. However, the use of such plants has been much more widespread outside the United States, especially in underdeveloped countries. None of these plants currently operate at reasonable costs.
The problem of adequate water supply has become more and more critical, since more human waste, as well as waste from agriculture and industries, pollute the world's freshwater resources, such as rivers, lakes, and groundwater. Some of the polluting (liquid) wastes can be treated by various technologies to become environmentally friendly, and thus preserving the water resources. The willingness of people to preserve nature by applying adequate technologies for treatment of waste, is confronted by considerable cost.